The graph shows composition-depth profiles of nitrided stainless steel AISI 316 as determined with glow discharge optical emission spectrometry (GDOES). It shows that, approaching from the austenite "substrate" for all nitriding potentials, a relatively steep increase in nitrogen content is established, after which the nitrogen content increases smoothly toward the surface.

Particularly for the higher nitriding potentials, the gradient in the nitrogen content increases again on approaching the surface. This phenomenon is largely explained by the strong concentration dependence of the diffusion coefficient of nitrogen.

The effect of the nitriding potential is also reflected in the hardness-depth profiles: a smooth decrease near the surface followed by a sharp drop at a position close to the transition from γN to γ. Nitrogen-expanded austenite is very hard, and the transition from the hardened case to the (soft) core occurs within a few microns.

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